Explorer EFI low speed hesitation?

[EBGeek]

Afternoon, all. Since I know I'm not the only bronco brother running an Explorer EFI swap, and I'd love to get some input before I keep throwing money at my issue.

Basically engine seems to studder/hesitate but only when taking off, once its going it pulls like no ones business. It only really seems to occur at or just above idle and it does seem to be more pronounced when its cold.

So far, I've ran fuel injector cleaner through it, replaced the inline filter (pump is external), cleaned and replaced the MAF sensor all to no avail (I also individually cleaned/tested each injector before I installed engine. I also ran it with a fuel pressure gauge on the rail and it stayed rock solid around 35psi regardless of throttle position. Does anyone have any other ideas to try before I go replacing costly fuel injectors and/or fuel pump? Everything I've read points to a fuel pump issue but my pressure shows ok, so I'm not sure...

 

[Timmy390]

What RPM is it idling at? What mods have been done to the engine? Any codes?

Rough idle? Have you cleaned or replaced the IAC?

Tim

 

[DirtDonk]

And just to be sure... You're sure there's no way it's a driveline vibration, rather than an unusual engine action?
Sounds like it's engine, since you said it's hesitating, but I wanted to ask anyway.

Since it's mostly when it's colder (does it go away completely when warm?) what about a temp sensor, or other temp related item such as a TFI module malfunction if you have a distributor? Or are you running the coil packs?
Nevermind, I see you're running OBD II so it must be coil packs.
Did Garry/EFI Guy do the harness?

Maybe even the IAC or TPS going wonky?

Vacuum leak too maybe? How's the seal on the PCV valve and how is it plumbed into the engine?

Got pics?

Paul

 

[Seventee]

What are you running for an air filter?

 

[EBGeek]

Great questions, all. Let me see if I can answer them, then I'll get a cold start video posted so you can hear what it's doing.

No mods other than Garry harness retrofit. It's currently idling at 600 I think, but it idles flawlessly (except it does kinda 'hunt' up and down for a sec before it settles into idle sometimes.

No codes, no other issues than the hesitation. I've cleaned the IAC bit have not yet replaced it. Wasn't sure if IAC could affect it this way.

Wonder if my O2 sensor is flaking out? Would it throw a code if it were out of range? It's just so odd that the stumble is only when you're coming off idle...

 

[Timmy390]

I would expect the idle to be a bit more say 700-750. Hope others come in on that.

O2 sensors should throw a code if reading is out of range.

IAC can cause off idle stumbles. You have a stock cam or bigger?

Tim

 

[DirtDonk]

Not 100% sure, but I think the 600 is more likely. Most of the modern cars I've driven in the last ten years or so actually idle around 500 or even a bit less.
More than that, like the older 700-750 range is hard on shifts with automatics.
Whatever the reason though, I think 600 is as high as you'd normally want to go.
Especially with a non-computer C4 maybe?

But it's a good thought to check that because of the off-idle issue.

Paul

 

[EBGeek]

Idle was higher when Garry first did it .. But it would jump when put into gear so I had him lower it some. It's been that way for 10s of thousands of miles, so I don't expect that's an issue.

Cam is stock Explorer. I just looked at IAC is in stock at local parts store so I may give that a shot, it's worth the $50 to test it. This is driving me nuts since it's always ran so perfectly, that's why I know something has changed.

@Paul - that's interesting input. I actually asked him to take the idle lower (since I like a butter smooth shift into gear) but he refused saying it was a low at it would go (and already lower than he prefers it)

 

[DirtDonk]

Ahh, good to know. Thanks for the info. Interesting though...
Might be due to the fact that the computer controls the automatics that are usually associated with the Explorer ECM (as in the 4R70W) and it can do something when shifting from Park or Neutral that reduces the effect. I wonder?

Paul

 

[DirtDonk]

Just another reason to hook one of my tach's up to the '68...

Paul

 

[Timmy390]

I just want and verified....Garry has me set to 750 at idle (manual trans). I added an idle air bypass due to my bigger cam needing more air at idle and that puts me 850ish to 900 depending on temps.


Tim

 

[DirtDonk]

Yep, sounds about right for a cam and a manual trans. Stock cam and auto seems like it would be lower.
With the higher idle there is less chance of bogging or stalling, but it plays heck with an auto trans sometimes. Such as trying to pull past the brakes when at a stop.
With 4.56 gears and 30" tall tires, that would be horrible on mine!

Paul

 

[Seventee]

The reason I asked about your air filter is I recently had a similar experience. I tried to do a quick test drive after a swap without the airbox installed and it exhibited all the same symptoms you describe, inconsistent idle, stumbling/hesitation off idle, but would smoke both rear tires once it got going. I was stumped because it ran perfectly in the donor, so I hooked up Forscan and the MAF readings were going crazy. I reinstalled the airbox and all is well.

Based on this I suspect your air filter arrangement or MAF sensor is causing you fits.

 

[Broncobowsher]

You mention 10s of thousands of miles.
Has it always had this stumble, or something new?

Which version of Explorer EFI are you running? That 35 PSI sounds a little low to me. if an early system with a return line and vacuum referenced pressure regulator I would expect to see a change as you get on the throttle and when you lift. If a late system no return, those ran a smaller injector but at a higher pressure to get the flow back up. I would only expect 35 PSI with a return style while pulling high manifold vacuum.

 

[EBGeek]

Hey Seventeen, I just have a K&N stuck on the end of the Explorer intake hoset. I've always wanted to do an airbox of some kind, but I can't seem to find a way with my intake in the passenger side.

Hey Bowsher, I might need to check the fuel pressure reading (since that's depending on my questionable memory) but I do recall it dancing 5-10lbs when you'd get on it vs let out, which I thought was normal behavior (but maybe it isn't). And yeah this is a new behavior after all these miles, that's why I know something has changed/failed...

I bought a replacement IAC to try, I'll post results here in case anyone is invested in my issue

 

[DirtDonk]

Are there fairly simple ways to test IAC’s and TPS units with maybe just an ohmmeter?

 

[4xfun]

The TPS can definatley be adjusted using a multimeter. You just need to get the specs for the explorer unit. I don't know if they changed much over the years, but is usually fairly simple to check and adjust. You need to get a set of back pins to check the voltage and resistance. This site explains how to check it, but I would verify the readings. I had a similar issue with an Xterra. https://my.prostreetonline.com/2015/12/03/how-to-test-a-ford-explorer-tps-sensor/

 

[surfer-b]

I havnt read through all this but chk the TPS with a meter

 

[EBGeek]

Well for those invested in my outcome - it was the IAC!!! I had cleaned it thoroughly when I installed the engine, but it had gotten a lot of build up on it over the years/miles. Its possible I could've cleaned it and it been ok, but replacing it completely removed the stumble/hesitation. 5 min and $60 later and I'm back at it. Thanks for all the input, everyone!

 

[DirtDonk]

Great to hear! Thanks for following up.